More cleanup

[lihaoyi]

• Break up mill.client into mill.client and mill.constants, only depend on constants through most of the codebase and only use client in runner.
  • The name constants sucks but the dependency graph is better, we can change the name later if we can think of a better one
• Interp#watch should live on MainModule rather than BaseModule, since ExternalModules should not need it
• MillBuildBootstrap should work with RootModule rather than BaseModule, since you never need to bootstrap ExternalModules
• Pull some stuff out of MainModule into separate files
• mill.define.Watchable moved to mill.define.internal.Watchable
• Made ModuleTask private and documented the hack
• Got rid of the Applicative.Applyer trait and inlined def ctx() where necessary
• Lots of ScalaDoc